1. Field of Invention
Aspects of the present invention are directed to the processing of an Orthogonal Frequency-Division Multiplexed signal, and more particularly to a system and method of synchronizing an FFT window on an Orthogonal Frequency-Division Multiplexed signal so that the FFT window includes substantially all of the useful data portion of a symbol and any appreciable echo energy lies within the guard interval of the symbol.
2. Discussion of Related Art
In Orthogonal Frequency-Division Multiplexing (OFDM) systems, information such as compressed audio and/or video data is carried via a large number of individual carriers (i.e., sub-carriers) in a frequency multiplex. The frequencies of the sub-carriers are selected so that the modulated data streams are orthogonal to each other, thereby eliminating cross-talk issues. Although each carrier transports only a relatively small amount of information, high data rates may be achieved by using a large number of carriers (e.g., 2048, 4096, 8192, respectively termed 2 k, 4 k, and 8 k mode) multiplexed together. The individual carriers are modulated (e.g., using phase-shift keying (PSK) techniques, or amplitude modulation techniques, such as Quadrature Amplitude Modulation (QAM)), with each carrier having a fixed phase and amplitude for a certain time duration, during which a small portion of the information is carried. That small portion of information is called a symbol, and the time period for which it lasts is called the symbol duration. The modulation is then changed and the next symbol carries the next portion of information. Examples of known OFDM systems include DVB-T (Digital Video Broadcasting-Terrestrial) Standard systems, T-DAB (Terrestrial Digital Audio Broadcasting) Standard systems, 3G and 4G mobile phone wireless network systems, as well as others.
In OFDM systems, modulation and demodulation are performed using the Inverse Fast Fourier Transformation (IFFT) and the Fast Fourier Transformation (FFT), respectively. The time duration of a symbol is the inverse of the carrier frequency spacing, thereby ensuring orthogonality between the carriers. In addition to the data that is carried by an OFDM signal, additional signals, termed ‘pilot signals’ (whose value and position are defined in the applicable standard, and are thus known by the receiver) are inserted into each block of data for measurement of channel conditions and also for synchronization.
In order to overcome inter-symbol interference, a portion of each symbol (e.g., the first portion or the last portion) is copied and appended to the beginning or end of the symbol. For example, in DVB-T standard systems, the last portion of the symbol is copied and appended to the beginning of the symbol as a cyclic prefix. In OFDM systems, and as used herein, that copied portion of the symbol is termed the “guard interval” and its duration (or length) is typically denoted Δ, the duration of the original symbol (i.e., the “useful symbol duration”) is typically denoted TU, and the increased symbol duration is typically denoted TS, where TS=TU+Δ. Provided that most (or ideally all) echo energy from a prior symbol falls within the guard interval, the symbol may still be recovered.
In an OFDM receiver, the received OFDM signal is demodulated to baseband using some type of quadrature amplitude demodulation or phase shift keying demodulation, the resultant baseband signals are then typically low-pass filtered, and the filtered baseband signals are then sampled and digitized using analog to digital converters (ADCs). After removal of the guard interval, the digitized signals are then provided to an FFT module and converted back to the frequency domain. Because of the presence of the guard interval, a nearly infinite number of options exist as to where to place the FFT window to evaluate the symbol. In general, it is desired to place the FFT window on the useful part of the symbol (TU), and so that all or nearly all echo energy lies within the guard interval (Δ) of the symbol. One known process for determining where to locate the FFT window is described in U.S. Pat. No. 6,459,744 B1 (hereinafter the '744 patent), which is incorporated by reference herein, and is now functionally described with respect to FIGS. 1, 2A, and 2B herein, and which correspond to FIGS. 7, 1, and 2 of the '744 patent, respectively.
As described in the '744 patent, a received time domain OFDM signal x(t) is sampled at a sampling frequency HS and converted into the frequency space by means of an N-point FFT 72. The sampled signal is also provided to a means 76 for measuring a correlation of the guard interval of the sampled signal. The means for measuring the correlation of the guard interval 76 includes a correlator and summing accumulator 761, that is provided with the sampled signal x(t) and with the same sampled signal x(t) delayed by the useful symbol length TU.
As described in the '744 patent, under ideal conditions where there is no noise, no multiple paths (i.e., no meaningful echo energy), and no co-channel interference, the correlation of the guard interval preceding the useful part of a symbol and the end of the useful part of the symbol may not only be used for a “rough” temporal synchronization, but may also be used for a fine temporal synchronization of the FFT window placement. This is illustrated in FIG. 1 of the '744 patent reproduced here as FIG. 2A.
FIG. 2A illustrates a measurement of the correlation Γx(Tn), and the pulse response h(t) 13, for both a noise-affected idealized signal 11 and for a noiseless idealized signal 12 that are received over a transmission channel with only one path 14 (i.e., having no meaningful echo energy). As can be seen in FIG. 2A, where there is no meaningful echo energy present, the correlation of the guard interval and the end of the useful part of the symbol will resemble a triangle with a defined peak in the region of maximum correlation.
However, as also described in the '744 patent, where there is significant echo energy present in the received signal, or where there is a high level of interference, the correlation of the guard interval and the end of the useful part of the symbol will be less well defined, and will resemble more of a deformed trapezoid, with each echo being reflected by a correlation peak. For example, FIG. 2B illustrates a measurement of the correlation Γx(Tn), and the pulse response h(t) 23, for both a noise-affected signal 21 and for a noiseless signal 22 that are received over a transmission channel having two paths 241, 242 spaced apart by a length of the guard interval Δ and received with identical power (i.e., a main signal having an echo with the same power as the original signal, and having a delay equal in length to the guard interval). As can be seen in FIG. 2B, the main signal and the echo are each reflected as a correlation peak, and although the correlation of the guard interval and the end of the useful part of the symbol may still be used to determine a length of the OFDM symbol, its precision makes it difficult to discern where to optimally place the FFT window.
To overcome this deficiency, the '744 patent describes the use of a means 77 for computing an estimation of the pulse response of the channel. As described in the '744 patent, after the received signal x(t) is sampled and converted into the frequency space by means of the N-point FFT 72, those samples corresponding to one or more reference carriers (i.e., pilot signals) are extracted and grouped to construct a fictitious synchronization symbol in module 771. The fictitious synchronization symbol is standardized by multiplication 772 and then subjected to an inverse FFT 775 on N/R points (where N represents the number of orthogonal sub-carriers and R represents the spacing of a reference sub-carrier every R sub-carriers) to provide an estimation of the pulse response (ĥn) of the channel.
The '744 patent describes that an analysis of the estimation of the pulse response of the channel may be used to determine the useful part of each symbol in the frame of the received OFDM signal, and to identify the location of the main signal and any significant echoes. However, the '744 patent notes that under certain circumstances, an analysis of the estimation of the pulse response of the channel is incapable of distinguishing between a long echo (e.g., an echo having a delay greater than TU/4 and less than TU/3 of the current FFT window position) and a pre-echo. To remove this ambiguity, the '744 proposes the use of the correlation of the guard interval to remove the ambiguity inherent in the estimation of the pulse response of the channel.
As shown in FIG. 1, and as described with respect to FIGS. 6A and 6B of the '744 patent, the '744 patent provides a signal processing means 75 that analyzes the spread of the correlation result provided by the means for measuring the guard interval correlation 76, and uses that information to distinguish between a pre-echo and a long echo in the estimation of the pulse response of the channel. As described in the '744 patent, provided that the receiver was previously well synchronized on the main path of the signal, the measurement of the correlation of the guard interval and the end of the useful part of the symbol is spread to a much greater extent in the case where the echo is a long echo than in the case of a pre-echo. Thus, by counting the number of samples that go beyond a given decision threshold, or by calculating the ratio of the number of samples greater than this threshold relative to the number of samples below the threshold, it is possible to distinguish between a pre-echo and long echo. Upon making a determination as to whether the echo is a long echo or a pre-echo, the signal processing means 75 uses that determination to adjust (i.e., delay or advance) the timing of the FFT window.
As noted above, the '744 patent discloses how an analysis of the estimation of the pulse response of the transmission channel may be used in conjunction with an analysis of the spread of the correlation of the guard interval and the end of the useful part of a symbol to adjust the location of the FFT window. However, the methodology used in the '744 patent presumes that the FFT window was previously well synchronized on the main path of the signal. In contrast, embodiments of the present invention are directed to systems and method for optimally locating an FFT window, irrespective of whether the FFT window was previously well synchronized.